39 flathead harmonic issue

[callbald]

just had the engine and transmission rebuilt and added a new clutch. The engine has what I call a harmonic (vibration) issue that starts in at about 1500-2000 rpm and you really feel it at 35-40 mph. It is not a timing or miss issue. I hardly know where to start sorting this out. The engine has plenty of power. The piston liners were removed and the block was rebored from a 221 to 239. Even at idle it is not glassy smooth as before.

[40cpe]

the first questions are: (1) was the engine balanced at rebuild? Obviously parts were used that weren't in the engine when it idled smoothly.(2) Was the pressure plate mounted on the flywheel and balanced as an assembly? The new pressure plate could be the issue if it is of questionable origin.

[rotorwrench]

You need to get a correlation of RPM to speed so you can see if it does it at that same RPM with the clutch disengaged. Also check with clutch engaged and in neutral. This should further isolate the cause. If it vibrates at a particular rpm with all else disengaged then it is in the engine, flywheel, & clutch cover relationship. From there the clutch cover would have to be removed to isolate the engine & flywheel relationship. If that is the case, the flywheel & crankshaft may need to be balanced.

Sometimes an odd piston gets installed that is a lot heavier than the others and that can throw balance out of whack. Clutch covers could be funky if they are off shore reproductions.

[callbald]

The engine was not balanced, it is an internally balanced engine. All components: were bought in sets. The thought came up about the clutch having finger blocks that did not get removed, but the clutch asy did not have them. resurfaced original pressure plate. I am now looking at the distributor/ignition wires. Picking low fruit first. Having to pull the engine to balance or rear drive train to get to clutch does not sit well.

[rotorwrench]

It could easily be rear drive train. If it is the engine, it will vibrate at that rpm any time it gets to that level. Rear axles, drive shaft support bearing, & transmission bearings/gears can all be eliminated easily. Clutch & flywheel not, so easy. Easy things first like you mentioned. If it only shakes going down the road, it could even be tires or wheel components.

[fordscript]

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I think that engine is coming out! When you removed the liners and bored it out to 239 ci, the larger pistons would be heavier than the 221. The crankshaft on a 239 is heavier, to work with the 239 pistons. Even though the pistons are a 'balanced' set, they will cause an imbalance due to their relationship with the 221 crank and rods. 239 rods are also heavier. Its quite involved.

[JSeery]

All of this is why I balance any engine I build. Much easier and cheaper to do it the first time! Sometimes you can get away with it and skip balancing, but they always run better (smoother) balanced so not worth it to me.

[rotorwrench]

Piston & rod weight can be critical when balancing the crank if there is a lot of difference from the original configuration. They weigh the components and whittle on them first to get them as close to the same as possible. The bob weights for the crank throws are set up to mimic the weight of the reciprocating assemblies before they even start rotating the crank for balance checks. There is a calculation process for this that can be found on line.

The crazy thing is, that sometimes you can get away without balancing just fine but there can be those less fortunate times too. You just never know.

[J Witt]

Make sure all the rubber motor mounts/transmission mount are in good condition and properly tight. Ditto for all the exhaust system hangers. If you have a sufficiently heavy component that will vibrate at a harmonic of an engine rpm, it can set up a really annoying noise and also can cause a component failure. It doesn't necessarily have to be the engine. To get a harmonic you just need a forcing stimulus and some other component that matches the frequency of the stimulus (like a violin bow causing the string to vibrate).

[Lawrie]

When I switched from the early skinny rods to the 21a rods on my 36 engine,the machine shop had to ADD weight to the crank to get it right,it ran as smooth as after.but the smoothest engine in my collection is the stock 33 babbit engine with standard bore ,
Lawrie

[JSeery]

Ford flatheads were a well balanced engine from the factory and seem to stay that way as long as compatible (similar to original weight) parts are used in a rebuild. As soon as different parts enter the picture it becomes more of a gamble. An overbore is going to change the rotating dynamics, as will a multitude of other modifications. Then it starts becoming more and more of a gamble without recalculating the balance requirements of all the rotating components and checking it.

The biggest area that can be an issue is the flywheel and pressure plate assembly. They have a lot of mass and a lot of distance from the rotational centerline. Flywheel/pressure plate assemblies almost always need to be balanced. A pressure plate will only be in balance in one location on the flywheel and needs to be marked so that if it is removed it can be replaced in the proper location to maintain it's balance.

[scooder]

If it was a tin can sleeved engine, it woulda/shoulda had the steel pistons. These are really light compared to stock Ford ally pistons. If the crank is the same one that came out of that engine, the new pistons have thrown it all to pot.
It really should have been balanced.
Sorry, but it sounds that you are gonna have to whip it out.
By all means, check all the other stuff first. Go through what rotorrench has said.
Martin.

[callbald]

I am forwarding all the threads to the engine rebuilder. On the motor mounts, it looks like they could be over torqued. what is the spec for tighting motor mounts?

[GOSFAST]

Hi "call", if it turns out to be an "imbalance" issue, the odds are in your favor that it will most likely be a flywheel/clutch problem and not an engine one.

This is NOT guaranteed, just from our own previous experiences.

Thanks, Gary in N.Y.

P.S. If we are correct here it is a relatively simple fix (not counting your labor) to balance the flywheel assembly. We balance them as a single unit, bolted together and "indexed" (alignment marks on the plate/wheel), for reassembly! This all but eliminates the need to do any welding (really not recommended) on the cover itself

[Seth Swoboda]

Originally Posted by callbald

The engine was not balanced, it is an internally balanced engine. All components: were bought in sets. The thought came up about the clutch having finger blocks that did not get removed, but the clutch asy did not have them. resurfaced original pressure plate. I am now looking at the distributor/ignition wires. Picking low fruit first. Having to pull the engine to balance or rear drive train to get to clutch does not sit well.

You should always have the rotating assembly balanced during an engine rebuild. These parts are not balanced right out of the box new. Your engine rebuilder, if competent, should know or have known this.

[Brendan]

i had a chebby garbage truck motor rebuilt, and they balanced. i had to look at the oil pressure to see if it was still running at a stop

[scooder]

Gary,
Just out of interest,
Why not balance the flywheel and clutch separately? This is how I get this done. I like the fact that I can fit any balanced clutch on and be good to go.
Martin.

[rotorwrench]

The flywheel could be balanced with the clutch cover if mounted on a purpose made mandrel. Since the cranks are pinned, it should go on all cranks the same way. The pins keep them concentric. We used to static balance the big Bell rotors on a Marvel stand and they needed little or no adjustment of balance after they were installed on the aircraft and dynamic balance was checked. Some aircraft had a bit of run out in the mast shaft though so that would require a touch of adjustment but never a lot.

[scooder]

Gary does his together, I have them done separately by choice.
Martin.

[GOSFAST]

Originally Posted by scooder

Gary,
Just out of interest,
Why not balance the flywheel and clutch separately? This is how I get this done. I like the fact that I can fit any balanced clutch on and be good to go.
Martin.

Hi Martin, couple reasons we prefer it as a single unit (when possible)?

Balancing a flywheel OR cover is considered "single-plane" balancing, unlike the cranks where there is a right and left side, which becomes "dual-plane" instead.

First, if you do them together (as a single piece) and index it you have much better control over the amount of very slight remaining imbalance, this would be a much lower number than doing both pieces separately.

Second, we have never been comfortable working on any covers separately, there's not much drilling room and we don't like "welding" on them at all for adding weight.

Now, having said all this, a "dependent" balanced unit (ex. OEM 400" SBC or a 454" BBC) can't be done this way. In these examples the flywheels MUST be mounted on the cranks during the procedure! In these instances we do the covers separately, not much choice otherwise.

I'm not even certain there is a "dedicated" fixture for the cover's only on a conventional engine balancer, if there is we don't have one? We do have the one for the flywheels however!

Thanks, Gary in N.Y.

P.S. To be really clear on this subject we recommend balancing ALL units that are built here. We take no mfr's at face value that their components are balanced correctly, we CANNOT afford the risk having to stand behind our builds. Again, speaking from a business standpoint, there is no way I "could or would" risk sending any units out of here that may have a single solitary chance of any issues. The idea of any customer on a build done here, possibly hundreds or thousands of miles away, having a single issue is more than I care or am willing to risk! We dyno just about EVERY build we assemble, once in a while with very select customers we let this go, rarely though.